Photonic crystals include periodically repeating internal regions of high and low dielectric constants. Photons propagate through the structure based upon the wavelength of the photons. Photons with wavelengths of light that are allowed to propagate through the structure are called “modes,” while photons with wavelengths of light that are not allowed to propagate are called “photonic band gaps.” The structure of the photonic crystals define allowed and forbidden electronic energy bands, resulting in spectral selectivity of light.